164 H. SAXTON BURR 



forebrain tracts are not present, but we do find the primary and 

 secondary olfactory tracts more or less well differentiated. 



We find, then, in the transplanted hemisphere the following 

 tracts and nuclei to be present. ,As in the normal hemisphere, 

 running caudad from the olfactory bulb is the tractus olfactorius 

 lateralis, some of whose fibers enter the anterior olfactory nucleus 

 and some of which proceed to the posterior pole (fig. 4) . There 

 is also a tractus olfactorius ventrolateralis running from the 

 olfactory bulb caudad to the anterior olfactory nucleus, where 

 apparently most of the fibers end (figs. 5 and 6). The lateral 

 forebrain tract, a large tract in the normal hemisphere, is, in 

 the transplanted hemisphere, reduced in size owing in part to 

 the absence of any ascending fibers (figs. 3 and 6). The fibers of 

 this lateral forebrain tract which are present in the transplanted 

 hemisphere start from the lateral olfactory nucleus and run 

 through the pars ventrolateralis hemisphaerii to the posterior 

 pole of the hemisphere and there end. Running from the olfac- 

 tory bulb ventromedially is the tractus olfactorius medialis. 

 This tract in the transplanted hemisphere starts as a fairly large 

 bundle of fibers running ventromedially into the region of the 

 nucleus medianus septi, where it becomes very much reduced 

 in size, finally to end. In an embryo killed thirty-seven days 

 after the operation, this tract runs caudally to the primordium 

 hippocampi. The two fiber tracts remaining undiscussed con- 

 necting the pallial region with the diencephalon are difficult to 

 unravel in the transplanted hemisphere. There is apparently 

 the beginning of a columna fornicis in a twenty-day embryo, 

 since a number of fibers can be traced from the anterior boundary 

 of the primordium hippocampi for a short distance ventrally 

 close to the ependyma of the ventricle. Of the stria medullaris 

 complex no trace was found. 



From the above findings it is evident that Herrick's division 

 of the hemisphere into four columns corresponding to the divisions 

 of the lower parts of the brain stem is entirely correct. For we 

 find that in transplanting the hemisphere we prevent the entrance 

 of any ascending fibers and as a result find the ventral half 

 of the hemisphere much less differentiated than normally (figs. 2 



